The present invention relates to using control functions to adapt a radio receiver to selectively perform either conventional or joint demodulation.
The cellular telephone industry has made phenomenal strides in commercial operations in the United States as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations. Solutions are required to meet these increasing capacity needs and to maintain high quality service.
As additional users are introduced to wireless cellular systems, the system characteristics change from a noise-limited to an interference-limited environment. Interference problems can arise when signals transmitted between locations suffer from echo distortion or multipath time dispersion caused, for example, by signal reflections from large buildings or nearby mountain ranges. Multipath dispersion occurs when a signal proceeds to the receiver along not one, but many paths, so that at the receiver many echoes having different and randomly varying delays and amplitudes occur. For example, when multi-path time dispersion is present in a TDMA system, the receiver receives a composite signal comprising multiple versions of the transmitted symbol that have propagated along different paths having different relative time delays. At the receiver, such multiple versions of the transmitted signal can cause self-interference.
In addition to being performance limited by self-interference, receiver performance also may be limited by co-channel signal interference. Work has been done to address the problem of co-channel signal interference by means of joint demodulation. Note, for example, the discussions of and approaches to joint demodulation that are found in U.S. Pat. Nos. 5,506,861, 5,790,549 and 5,790,606, assigned to the assignee of the present invention and the teachings of which are incorporated herein by reference.
The present invention relates to a radio receiver comprising a Doppler spread estimator for generating an estimated Doppler spread value of the carrier; a first estimator generates an estimated carrier-to-dominant interferer plus noise ratio value, and a second estimator generates an estimated dominant interferer plus noise-to-noise ratio value. In addition, a control unit selectively controls the radio receiver, in accordance with the estimated values, to selectively implement either single-user demodulation of the carrier or joint demodulation of the carrier and dominant interferer.
The invention also contemplates a method of operating a radio receiver, which receives a signal that includes a desired carrier, a co-channel dominant interferer and noise. The method comprises generating an estimated Doppler spread value of the carrier; developing an estimated carrier-to-dominant interferer plus noise ratio value, and providing an estimated dominant interferer plus noise-to-noise ratio value. In addition, the method controls the radio receiver, in accordance with the estimated values, to selectively implement either single-user demodulation of the carrier or joint demodulation of the carrier and dominant interferer.
The foregoing and other advantages and features of the invention will become apparent upon a consideration of the following description, when taken in conjunction with the accompanying drawings.